DESCRIPTION: The proposed research tests theoretical and methodological aspects of the Polyvagal Theory. The Polyvagal Theory proposes that the evolution of the mammalian nervous system provides the neurophysiological substrate for the emotional experiences and affective processes that are a major component of social behavior. The theory proposes an integrated Social Engagement System that includes both the phylogenetically most recent adaptation of the autonomic nervous system (i.e., the myelinated vagus that functions as the vagal brake) and the cranial nerve regulation of the striated muscles of the head. According to the theory, appropriate social behavior is a product of an active vagal brake and an active somatomotor component. When either one or both of the components is deficient, less appropriate social behavior will be observed. Two experiments are proposed to test hypotheses derived from the theory to investigate how expressions of components of the Social Engagement System may be related to social behavior. Study 1 uses a cross-sectional design to maximize sample size, estimate developmental trends, and investigate several methodological hypotheses including issues of test-retest reliability of measures of vagal activity. Study 2 uses a longitudinal design to address developmental and individual differences questions. Questions of intra-individual development as well as the usefulness of early measures of visceromotor and somatomotor components in the prediction of kindergarten and first grade measures of social behavior are addressed. Specific aims are: 1) To examine the relation between the dynamic regulation of the vagal brake, cortisol activity, and social-emotional behavior in young children; 2) To examine age-related differences in the ability to recruit and integrate components of the Social Engagement System; and 3) To evaluate whether measures of the Social Engagement System assessed during preschool are related to individual differences in behavior and socialization in kindergarten and first grade.